1. Field of the Invention
The present invention relates to a developing apparatus by which a latent image on a latent image carrier is made visible using a two-component developer and also to an image forming apparatus, of an electrophotographic system or an electrostatic recording system, which performs image formation on a latent image carrier, such as a photoreceptor, and a dielectric substance using the above developing apparatus.
Recently, the dimensions of printers, which are image forming apparatus, have been reduced. In accordance with the reduction in the dimensions, a compact screw type developing apparatus has been used because of the highly efficient stirring and conveyance properties thereof.
2. Description of the Related Art
A developer used in a known screw type developing apparatus is composed of carrier particles, the particle size of which is 60 .mu.m, and toner particles, the particle size of which is 10 .mu.m, wherein the mixing ratio of toner to carrier is 4 to 6 weight %. A known developing roller includes a magnetic roller having a plurality of fixed magnetic poles; and a cylindrical rotational sleeve made of non-magnetic material, wherein the magnet roller is provided in the cylindrical rotational sleeve. Around the developing roller, there is provided a metallic doctor blade which scrapes away surplus developer so that a toner layer of a predetermined thickness can be formed on the developing roller, and also a metallic recovery blade which scrapes and recovers the developer from the developing roller after development.
Screws and paddles are implementations of stirring and conveying means for triboelectrically charging toner and carrier and supplying them to the developing roller. The screws are spiral screws made of resin, disposed in parallel with the shaft of the developing roller; and a partition plate is provided between the two screws.
There are provided two types of spiral screws. One is a spiral screw provided with a spiral blade and the other is a spiral screw provided with a spiral blade.
The spiral type screws are provided in such a manner that two screws are disposed in parallel. In order to stir and convey the developer between the two, they are driven reversely (i.e., oppositely), relatively to each other.
When one of the spiral type screws is rotated in one direction, the developer is taken from a supply adjacent a lower portion of the spiral type screw and discharged from an upper portion. In this way, the developer is conveyed from the spiral type screw to the developing roller.
Paddles are made of the same material as that of the screws. The paddles are composed of several thin plate-shaped blades provided at both ends of the shafts of the screws. These paddles have the function of extruding and conveying the developer so that the developer cannot remain at the ends of the two screws.
The screws are rotational bodies rotated when torque is transmitted to the end portions of the shafts from a drive system. Accordingly, there is provided a gap between the outer diameters (blades) of the screws and the bottom portion of the developer accommodating container. Therefore, sometimes the developer remains in that bottom portion of the container without being conveyed.
Accordingly, in the case of a small developing apparatus in which only a small amount of developer is accommodated, when even only a minute amount of developer remains in the gap, all the developer cannot be used, which causes a problem.
In the conventional screw developing apparatus, the developer stirring and conveying efficiency of the screws is lowered due to the gap, so that the quality of the developer deteriorates. That is, when the developer remains in the gap, an absolute amount of carrier used for toner conveyance is reduced, so that loads are given to carrier particles due to abrasion and shock. Accordingly, toner filming occurs very early on the surfaces on carrier particles, and the surface resin layers of the carrier particles are early worn away, which deteriorates the durability of carrier.
In the case of the conventional screw type developing apparatus, when the used developer of which the life has expired is discharged from the discharging port provided in the developer accommodating container, an amount of the used developer of about 70 g remains in the developer accommodating container in which the developer of about 700 g can be accommodated.
Accordingly, even when the used developer is discharged and new developer is supplied, the carrier is used, the toner conveyance properties of which are deteriorated due to the surface abrasion of remaining developer and toner filming. Consequently, in the image forming apparatus provided with the conventional screw developing apparatus, even after the developer has been replaced, the problems of uneven density and whitening are caused in the formed image.